Thermosensitive resistance element containing eutectic mixtures



Feb. 3, 1953 c. l. HALL THERMOSENSITIVE RESISTANCE ELEMENT CONTAININGEUTECTIC MIXTURES Filed Jan. 6, 1950 4 Sheets-Sheet 1 Pig. 2.

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RESISTANCE ELEMENT Feb. 3, 1953 c.

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I mventor: Chester 1. Hall,

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His Attorney Patented Feb. 3, 1953 THERMOSENSITIVE RESISTANCE ELEMENTCONTAINING EUTECTIC MIXTURES Chester 1. Hall, Vischers Ferry, N. Y.,assignor to Genera-1 Electric Company, a corporation of New YorkApplication January 6, 1950, Serial No. 137,233

1 Claim.

This invention relates to thermcsensitive materials. More particularly,it relates to new and useful thermosensitive materials provided byeutectic mixtures of chemically stable salts having pronounced negativeresistance-temperature characteristics.

It is known that many inorganic salts are thermosensitive or havenegative resistance-temperature characteristics and decrease inresistance as their temperature is raised. These characteristics havebeen utilized to electrically indicate temperatures and control variousdevices and systems.

The use of simple inorganic salts or compounds as thermosensitivematerials is accompanied by a number of disadvantages which detractseriously from their usefulness. One of these disadvantages is that atall temperatures except those near or at their melting point the rate ofchange of resistance with temperature of most simple salts is so gradualthat in order to operate a relay, for example, and through it control anelectrical circuit or system, the relay must be extremely sensitive andhence expensive to manufacture. With many salts or compounds, the changein resistance even at the melting point is so small as to require anextremely sensitive relay to react to the change. In addition, mostsalts melt at temperatures which are higher than the range oftemperatures usually encountered i in many devices or systems which itis desired to control or in which it is desired to give an indication ofcertain specific temperature conditions by electrical means.

It is an object of this invention to provide intemperaturecharacteristics of various chemical salts and their eutectics, Fig. 11is a cross-sectional view of a typical thermosensitive element using thepresentmaterials, and Fig. 12 shows a typical control or indicatingelectrical circuit utilizing the present materials in a thermosen-.sitive element.

It has been found that thermosensitive materials having a high negativerate of change of resistance with a slight change in temperature may beprovided by a eutectic mixture of chemically stable salts.

More particularly, it has been found that such thermosensitive materialsmay be provided by a eutectic mixture of chemically stable salts havingboiling points which are relatively high compared to their meltingpoints and which are nonreactive with electrode and sheathing materialsused.

By a suitable choice of salts, a series of eutectic mixtures whoseresistance will decrease precipitately at different temperatures may beprepared and used in thermosensitive elements which are extremelysensitive to a very slight change in temperature.

The following are given as typical examples of the pronounced behaviorof eutectic mixtures of chemically stable salts as compared with thebehavior of their individual constituents. These combinations arepresented as indicative of the behavior of eutectic mixtures ofchemically stable salts in general and are not to be considered aslimiting the scope of the present invention.

Example 1 A eutectic mixture of 75%, by weight, thallous nitrate and25%, by weight, silver nitrate was prepared which melted at 83 C. Thislow melting point compares with a melting point of 206 C. for thallousnitrate and 212 C. for silver nitrate. As shown in Fig. 1, the thallousnitratesilver nitrate eutectic has a very steep negativeresistance-temperature curve in the vicinity of 80 C., whereas such acharacteristic is available in the individual constituents only at amuch higher temperature. The extreme steepness of the eutecticresistance curve at about 80 C. representing a change of over twodecades (from 25,000 ohms to 100 ohms) from about 75 to 85 C. makespossible the use of a relativelyinsensitive and inexpensive relay deviceto signal approximately an 80 C. temperaturecondition in any desiredinstallation or to actuate controls at that temperature. On the otherhand, the resistance of silver nitrate alone drops sharply only at over200 C. as does that of thallous nitrate. There is thus provided aeutectic mixture of these salts which is very thermosensitive at a'muchlower temperature than either of its constituents.

Example 2 A eutectic mixture of 50%, by weight, cuprous chloride and50%, by weight, thallium mono- .ual decrease in resistance whilethallium monochloride decreases gradually and then more sharply but at amuch higher temperature and in a much lower resistance range.

Example 3;

Reference to Fig. 3 shows that while rubidium chloride, melting at 715C., and cuprous chloride, melting at 422 C., have a gradual decrease ofresistance with temperature in the usable resistance range, the eutecticmixture of these two salts exhibits a very sharp drop in resistance inthe vicinity of 150C. of over two decades in about twelve degrees from150 C. to. 162 C. This eutectic consists. of 40%, by weight, rubidiumchloride and 60%, by weight, cuprous chloride and melts at 150 C.

Example 4 A eutectic mixture of 23.25%, by weight, cuprous. chloride and76.75%, by weight, stannous chloride. melting at 172 C.. exhibits arelatively sharp drop in resistance; at. about 170 C.,. as shown in Fig.4, of approximately one decade in ten degrees from165 C. to 175 C. whichmakes it useful for causing a change in an. electricalcircult at thistemperature. This compares with a very gradual drop in resistance for.either of the constituents alone-inthis as well as higher temperatureranges.

Example 5 A eutectic mixture of 81.5%, by weight, beryllium fluoride and18.5%, by weight, lithium fluoride with a melting point of 420 C. wasprepared. This mixture exhibited a pronounced drop in resistance atbetween 260 C. and 300" C. with the sharpest decrease at about 295 C.for a drop of four decades in forty degrees. On the other hand, thelithium fluoride constituent had a very gradual decrease in resistancewith temperature as shown in Fig. 5 as does beryllium fluoride.

Example 6 As pointed out heretofore, rubidium chloride, melting point715 C., has a rather gradual drop of resistance with temperature. Silverchloride,

melting point 455 C., also exhibits a slow change A eutectic mixtureoi'80.%,.by-weigl1t, rubidium chloride and 20%, by weight, lithiumchloride melting at 318 C. exhibits. a very sharp drop in resistance atabout 290 C. of over one decade within ten degrees from 280 C. to 290 C.This is in contrast to the decrease of. resistance with temperatureexhibited by lithium chloride alone 4 which melts at 613 C. and rubidiumchloride which melts at 715 C. The contrast between the eutectic mixtureand the individual constituents is best shown in Fig. 7 which shows agradual drop in resistance with increase in temperature as compared tothe eutectic.

Example 8 As shown in Fig. 8, lead chloride (PbClz), melting at 501 C.,and silver chloride, melting at 455 C.,, show a gradual. drop inresistance with increase of temperature at around 300 C. However, theeutectic of 57.15%, by weight, lead chloride (PbClz) and 42.85%, byweight, silver chloride, melting at310 C., exhibits a sharp decrease ofabout over one decade in resistance in about ten degrees; at about 310C. or from 300 C. to 310 C. which makes it useful for indicating atemperature of this magnitude.

Example 9 Whereas, as. shown in Fig. 9, potassium chloride, melting at776 C., and. lithium chloride, melting at 613 C., show a rather sharpdecline in resistance near their melting point, their eutectic mixture,44% by weight potassium chloride and 56% by weight lithium chloride,melt ing. at 355 C., exhibits an even sharper decrease at. the muchlower temperature of about 370 C. The. resistance drop is about over onedecade in approximately twenty degrees from about 360 C. to 380 C. Thischaracteristic makesthiseutectic useful for indicating temperatures inthis latter range.

Example 10 The eutectic consisting of 48%, by weight, p0 tassiumfluoride and 52%, by weight, lithium fluoride melting at 530 C. isuseful for indicating; temperatures of about 520 C. by reason of its,sharp decrease in resistance at around this temperature as shown in Fig.10. This decrease is approximately one decade in twenty degrees fromabout 500 C. to 520 C. In contrast to this phenomenon, lithium fluoride,melting at 870 C., has a gradual decrease in resistance with temperature even around its melting point, whereas potassium fluoride,melting at 880 C., has a sharp decrease in resistance only at the muchhigher'temperature of about 850 C. as compared to 520 C. fortheeutectic.

From the above examples it will be seen that the eutectics of chemicallystable salts offer definite advantages over the individual salts astemperature indicators. In general, the eutectic mixtures exhibit a muchsharper decrease in resistance at temperatures at or near their meltingpoints than do the single salts. This makes possible the use ofrelatively insensitive relay or other resistance reactant instruments orcontrols which it is desired to. operate at the particular temperature.On the other hand, very sensitive and much more expensive relays arerequired to react at a certain specific temperature when the resistanceis decreasing onlygradually at. that point. This is true of" the simplesalts except in certain cases where the. resistance drop is greataround. their melting points. However, in gen eral, the melting pointsof chemically stable salts are relatively highso that at lowertemperatures the large. drop in resistance with temperature cannot. beutilized- However, as shown above, in the case of eutectic mixtures the,sharp decrease in resistance. occurs at lower temperatures. By a properselection. of eutectic mixtures, of chemically stable, non corrosivesalts, thermosensitive 5 materials each reacting at a specifictemperature in a wide range may be made available. Thus, eutecticmixtures as herein described may be used as the thermosensitive elementin heating system thermostats, bearing heat detectors, fire detectorsand the like.

In order that best results may be attained, the resistance of thepresent eutectic mixtures should exhibit a resistance drop of at leastone decade in five degrees at temperatures up to 150 C. For those whosesharp resistance drop occurs between 150 C. and 300 0., the drop shouldbe at least about one decade in ten degrees, and for those exhibitingthe rapid decrease above 300 0., the drop should be at least about onedecade in twenty degrees. This insures the rapid and certain actuationof relatively insensitive and inexpensive electrical circuits.

Fig. 11 shows a simplified thermosensitive element of the type which maybe used in conjunction with the thermosensitive materials disclosedherein. The element 1 comprises a tube 2 closed at one end which mayserve as one electrode. The second electrode 3 may be in the form of arod or strip of metal or other suitable electrically conducting materialwhich extends into and is spaced from the other electrode 2. Theintervening space between the electrodes may be filled with any of thethermosensitive materials 4 of this invention and a leak-proof plug 5 ofsuitable insulating ma- 2 terial placed at the open end of the tubeelectrode 2 and around electrode 3 to insulate the electrodes from oneanother and hold them in fixed relative positions as shown. Lead wire 8may be used to connect electrode 3 in circuit, and electrode 2 mayconveniently be so connected by means of a threaded collar 1 fixed toit. It will be understood, of course, that other means of connecting theelectrodes in an electrical circuit may be resorted to without departingfrom the scope and spirit of this invention. For example, lead wires maybe led from both electrodes or the necessary contact may be made byemploying a socket-fitting arrangement such as is used on theconventional electric light bulb.

A thermosensitive element such as described may be used for a number ofpurposes. It may be used as a fuel control, for example, in an oil orgas-fired furnace. An element containing a thermosensitive materialhaving a low resistance at flame temperature might be placed in contactwith the pilot flame of such a furnace. As long as the fuel supply is onand the pilot flame lit, the material would have certain resistancewhich would act through an electric relay system to regulate the mainfuel feed. If the pilot flame fails, the material in the element wouldassume a sharply different resistance which could be utilized to soaffect the current in the electrical system as to shut off the fuelsupply. The thermosensitive element could also be used to detect fires,detect hot boxes in railroad car trucks, give an indication of hot motorbearings and in general actuate a visual or audible signal whenever suchis desired to indicate a change in temperature in a particular range orto control the system at a particular temperature.

A typical electrical circuit in which the thermosensitive materials ofthe present invention may be used is shown in Fig. 12. Thermosensitiveelement 1 may be connected in series with a power source and relay 9,the latter being connected to the indicating or control means l0.Thermosensitive element I may be placed at any point in a device orapparatus where the temperature condition which it is desired toindicate or control exists. Thus, the element may be placed in a railwayor motor bearing in a heating thermostat system, a fire detection systemor the like, the thermosensitive material in the element being selectedto give a sharp decrease in resistance at the temperature at which thecircuit is to be actuated. When the thermosensitive material in theelement reacts, relay 9 is actuated and the indicating or control meansI 0 actuated. The indicating means may be an electric light or aloudspeaker or other audible means which signals the temperaturecondition to which the element 1 is sensitive. Alternatively, it may bea controlling means which, upon the critical temperature being reached,will cut ofi the power to the apparatus or device. The controls may alsobe arranged to turn on a power supply at a particular temperature. Thus,in the case of heating thermostats, when the critical temperature isreached, the heating means might be shut oil. In a railway bearingapplication, usually a visual or audible signal would be used to warnthe trainmen of the condition to be corrected. In a motor bearing, botha visual and audible signal to warn of the condition and controllingmeans to shut off the motor might be used. In a fire detection system, avisual and audible signal might be used and, additionally, means to turnon a fire extinguishing system.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

A thermosensitive electrical resistance element composed of a tubularelectrode closed at one end, a second electrode extending into the otherend of said tubular electrode and spaced therefrom by a plug ofinsulating material sealing said other end of the tubular electrode, anda body of resistance material within said tubular electrode inelectrical contact with both of said -electrodes,said body of resistancematerial consisting of a eutectic mixture of chemically stable saltshaving a rate of change of resistance in the unfused state in excess ofthat of the single constituent salts, said eutectic mixture consistingof one of the combinations of salts, thallous nitrate-silver nitrate,cuprous chloride-thallium monochloride, rubidium chloride-cuprouschloride, cuprous chloridestannous chloride, beryllium fluoride-lithiumfluoride, rubidium chloride-silver chloride, rubidium chloride-lithiumchloride, lead chloride-silver chloride, potassium chloride-lithiumchloride, and potassium fluoride-lithium fluoride.

CHESTER I. HALL.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 684,296 Nernst Oct. 8, 1901946,542 Garton Jan. 18, 1910 2,280,673 Thomas Apr. 21, 1942 2,480,166Schwartzwalder Aug. 30, 1949 OTHER REFERENCES International CriticalTables of Numerical Data, Physics, Chemistry and Technology, vol. 6,pages 150-151.

